A 2.4GHz WiMAX direct conversion transmitter

The purpose of this application note by Cecile Masse and Qui Luu is to demonstrate the Analog Devices' WiMAX 5V transmit signal chain for applications extending up to 2.7GHz. As the wireless communication industry moves toward higher RF frequencies, and higher data rates through wider modulation bandwidths, high performance linear transmit chains are required. WiMAX wireless broadband networks reflect such a trend. Deployment has started in the 2.5GHz and 3.5GHz bands for point-to-point and point-to-multipoint fixed applications. Data rates of up to 80Mbit/s are achieved using wideband OFDM modulations.

802.16 WiMAX fixed or mobile standards are based on 2N-carrier OFDM modulation: 256 for 802.16d and 512 to 2048 for 802.16e. Each of the 2N subcarriers can be modulated with either a QPSK, a 16 QAM, or a 64 QAM data sequence.

The standards also support different signal bandwidths, from 1.25MHz to 20MHz to accommodate variable rates, although the current profiles define channel bandwidths from 5MHz to 10MHz. The OFDM composite signal envelope amplitude can exhibit significant peaks and valleys, with a modulation depth close to 100 percent and peak-to-average ratio of about 10dB. This imposes severe linearity requirements on the transmit chain. To address these challenges, direct conversion architecture has been chosen. For this particular analysis, a full Tx signal chain, starting from the baseband signal generation, up to the voltage controlled amplifier and power detector functions (but excluding the power amplifier) was evaluated. The primary focus is the wireless broadband (WiBro) frequency band, 2.3GHz to 2.4GHz, used in Korea for the deployment of the 802.16d (fixed) and 802.16e (mobile) standards. However, this signal chain may also be used up to 2.7GHz.

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